The mid-Cretaceous (Barremian–Turonian [125–90 Ma]) Earth experienced some of the warmest temperatures and lowest thermal gradients of the entire Phanerozoic Eon. This time interval, therefore, represents one of the best ancient approximations of "greenhouse" climate. The Late Cretaceous was characterized by significant global cooling, but available oxygen isotopic records differ on the exact timing of the end of the "greenhouse" conditions. Records from DSDP Site 511 on the Falkland Plateau, South Atlantic (Huber et al., 1995), and the chalk from England (Jenkyns et al., 1994) suggest that peak warmth occurred in the early Turonian, at ~90 Ma (Fig. F8). Data from Shatsky Rise DSDP sites (e.g., Douglas and Savin, 1975; Savin 1977), however, indicate that peak "greenhouse" conditions existed in the Albian, at ~105 Ma. In addition, these stratigraphies differ on whether peak warming was immediately followed by long-term cooling (English chalk) or sustained warmth then cooling beginning in the mid-Campanian (Site 511 data). Differences between the various records may reflect real latitudinal climatic variations or diagenetic alteration of stable isotopic proxies.

There is also significant disparity as to exactly how much cooling occurred in the Late Cretaceous, especially in the tropics. Savin (1977) and D'Hondt and Arthur (1996) concluded that the Maastrichtian was characterized by surprisingly cool tropical SSTs (20°–21°C) based on ¹⁸O analyses of planktonic foraminifers, the "cool tropics paradox" (D'Hondt and Arthur, 1996). Wilson and Opdyke (1996), on the other hand, measured ¹⁸O values on rudists recovered from Pacific guyots and concluded that tropical SSTs in the same interval were extremely warm (between 27°C and 32°C). More recently, Pearson et al. (2001) have also found evidence for high Maastrichtian temperatures in planktonic foraminifers from clay units in Tanzania. The climate history of the Cretaceous is based on a limited number of oxygen isotope data points from few sites with little information from the tropics. In fact, Shatsky Rise Site 305 (Douglas and Savin, 1975) is among a handful of low-latitude sites that form the basis of most Cretaceous thermal gradient estimates that are used as inputs in climate models (e.g., Barron and Peterson, 1991).

There is a limited understanding of the evolution of bottom-water circulation in the mid- and Late Cretaceous, in particular, of how and when the transition from low-latitude (e.g., Brass et al., 1982) to high-latitude (e.g., Zachos et al., 1993) deep-water sources took place. Benthic foraminiferal ¹⁸O records are even sparser than those based on planktonic foraminifers, and there are very few benthic data from the entire Pacific. Thus, the role of this giant basin in the evolution of deep waters during the mid- and Late Cretaceous is poorly understood.

The long-term cooling of the Late Cretaceous was interrupted by a significant event in the mid-Maastrichtian, when the source of deep waters appears to have changed abruptly from low- to high-latitude sources (e.g., MacLeod and Huber, 1996; Barrera et al., 1997; Frank and Arthur, 1999). This event appears to have coincided with the extinction of the inoceramid bivalves (MacLeod et al., 1996) and possibly also the rudistid bivalves (Johnson et al., 1996). Growing evidence, however, suggests that this benthic event is distinctly diachronous (MacLeod et al., 1996). The change to high-latitude deep-water sources appears to have been long-lived, lasting until the LPTM. However, more benthic data are required to accurately characterize Late Cretaceous and Paleocene deep-water properties.

The Upper Cretaceous sections recovered during Leg 198 will help us determine (1) whether peak "greenhouse" conditions occurred in the Albian (Savin, 1977) or the early Turonian (Jenkyns et al., 1994; Huber et al., 1995); (2) if peak warming was immediately followed by long-term cooling (Jenkyns et al., 1994) or by sustained warmth and then by cooling beginning in the mid-Campanian (Huber et al., 1995), or whether cooling history varied between latitudes; (3) whether apparent cool tropical temperatures in the Maastrichtian (the "cool tropics paradox" of D'Hondt and Arthur [1996]) were real or the result of diagenetic alteration of planktonic foraminiferal tests; (4) the properties and age of mid- and Late Cretaceous deep water and from what oceanic region it was derived; (5) the timing and rate of changes in the sources of deep waters from low- to high-latitude sources; (6) the changes in deep-water mass properties that accompanied the MME and their effect on benthic faunas; (7) whether the MME led to a permanent change in deep-water source; and (8) the changes in vertical thermal gradients through time and whether climate and deep-water circulational changes were coupled.